Abstract Developing affinity reagents recognizing and modulating GPCR function by traditional animal immunization or in vitro screening methods is challenging. Some anti-GPCR antibodies exist on the market, but the success rate of development is still poor compared with antibodies targeting soluble or peripherally anchored proteins. More importantly, the majority of these antibodies do not modulate GPCR function. The current pipeline for antibody development primarily screens for affinity rather than epitope recognition. We propose to develop a new strategy utilizing natural ligand affinity to generate a library of antibody variants with inherent bias toward the active site of the GPCR. Instead of using phage libraries displaying antibodies with random CDR sequences, we will generate focused antibody libraries with a natural ligand encoded within or cross-linked to one of the CDRs or the N-terminus. We will limit randomization of the antibody to amino acids in regions around the ligand while leaving the ligand carrying part unaltered to tailor binding to the active site. A second round of randomization of the ligand carrying part will then be performed to eliminate the bias of the ligand. If successful, the proposed pipeline will enable the rapid generation of functional antibodies (both agonists and antagonists) against high value targets with poor epitope exposures including GPCR and other integral membrane proteins.